Technological development and demand for mobile devices have increased, giving rise to exponentially increased demand for batteries as energy source. Accordingly, increased number of researches have been conducted, focusing on the batteries that can meet a variety of demands.
For example, in terms of the shape of batteries, demand is high for prismatic secondary batteries or pouch-shaped secondary batteries thin enough to be employed in products such as cellular phones. In terms of the material for batteries, on the other hand, demand is high for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which provides advantage such as high energy density, discharge voltage, output stability, and so on.
Further, the secondary battery may be classified according to positive electrode/separator/negative electrode structure of an electrode assembly. For example, the jelly-roll type electrode assembly has a structure in which long sheets of positive electrodes and negative electrodes with a separator interposed therebetween are wound together, and the stack type electrode assembly has a structure in which a plurality of positive electrodes and negative electrodes, which are cut in a certain size unit, are sequentially stacked while having the separator interposed therebetween, and the stack/folding type electrode assembly has a structure in which bi-cells or full cells having positive electrodes and negative electrodes of certain unit and intervened by separator are wound with separator sheets.
The lithium secondary battery uses metal oxide such as LiCoO2 or the like as a positive electrode active material and a carbonaceous material as a negative electrode active material, and is prepared by placing a polyolefin-based porous separator between a negative electrode and a positive electrode, and providing a non-aqueous electrolyte containing lithium salt such as LiPF6 or the like. During charging, lithium ions of the positive electrode active material are released and intercalated into a carbon layer of the negative electrode. Conversely, i.e., during discharge, lithium ions of the negative electrode carbon layer are released and intercalated into the positive electrode active material. At this time, the non-aqueous electrolyte serves as a medium through which the lithium ions migrate between the negative electrode and the positive electrode. Such lithium secondary battery is charged and discharged as the lithium ions of the positive electrode are repeatedly intercalated and deintercalated to and from the negative electrode.
An electrode assembly having positive electrode/separator/negative electrode may be basically formed as a simple stack structure. On the other hand, the electrode assembly may be formed as a structure in which a plurality of electrodes (positive electrodes and negative electrodes), which are intervened by the separators, are stacked and bonded together by heating/pressing. In this case, bonding of the electrodes and the separators is achieved by heating/pressing, while the adhesive layers formed on the separator and the electrodes are faced each other.
In order to enhance adhesion of the separator to the electrode and thus solve a safety problem, it has been recently suggested to use an organic/inorganic composite separator coated with a slurry containing an excess amount of inorganic particles and binder polymer on a separator substrate.
Because the coating layer of the organic/inorganic composite separator is composed of an excess amount of the inorganic particles that takes most part, it may be effective in solving a safety problem caused by nail penetration, or the like. However, due to a relatively lower content of the binder polymer, a desired adhesion to electrodes is not provided. Further, while the content of the binder polymer may be increased, it may cause a problem that the same level of safety as provided by the configuration mentioned above cannot be provided.
Meanwhile, as one of general methods for coating the coating layer on a surface of the separator, the dip coating may be performed, by impregnating the separator sheets in a mixed solution of binder and inorganic components dispersed therein to form coating layer.
However, such dip coating has limitations in view of working speed, and it is also difficult to completely isolate a dipping device from outside, allowing solvent in the dipping device to continuously evaporate during the process and thus causing changes in the solids and deteriorated uniformity of the coating layer. It is also necessary to ensure that the process is kept under the constant condition.
Accordingly, there is a pressing need for a separator that can simultaneously improve both its adhesion to electrode and safety, and a preparation method of separator that can maximize productivity and also improve uniformity of the coating layer.